In the process of completing an oil or gas well, it is common to lower tool strings into the well on long lengths of tubing, typically from a coiled roll. The tubing serves not only to support the weight of the string of tools in the well, but also to transmit pressure from the surface of the well for activating the tools to perform various functions, such as sealing the well bore or perforating the well casing for access to product-bearing deposits. Such systems are sometimes referred to as tubing-conveyed perforating systems, or "TCP" systems.
Downhole ballistic tools of such systems are typically detonated by an attached firing head which, in the case of TCP systems, is hydraulically activated from the surface of the well. Good examples of such hydraulically activated TCP firing heads are disclosed by Huber et al. in U.S. Pat. No. 5,505,261, issued Apr. 9, 1996 and incorporated herein by reference as if entirely set forth. Huber et al. discloses three examples of TCP firing heads referred to as Circulation Direction Firing (CDF), Circulation Ball Firing (CBF) and Ball-actuated Circulation Firing (BCF) heads, each of which receives tubing pressure from the surface of the well to activate the firing head to detonate an associated tool.
Several of such firing heads have been successfully employed in field operations and have been received favorably in the market.
Because of the cost advantages of performing as many functions as possible with one trip down the well, several attempts have been made to combine multiple firing heads into a single string to be remotely operated by tubing pressure. Among these is a system described by Burleson et al. in U.S. Pat. No. 5,287,924, which discloses selective communication devices between each of the firing heads in the string.
Another such system is described by Edwards et al. in U.S. Ser. No. 08/752,810, filed Nov. 20, 1996, entitled "Device and Method for Performing Downhole Functions", which employs hydro-mechanical locks associated with each firing head.